The Liberal Democrat leader, Charles Kennedy, resigned this afternoon after reluctantly accepting he'd lost the support of his parliamentary party at Westminster.

Mr Kennedy brought his seven-year reign as leader of his party to an end with immediate effect and withdrew from the leadership contest he'd triggered 48 hours ago in a statement given at 3pm at the Liberal Democrat headquarters.

His resignation followed a dramatic admission in his earlier statement on Thursday that he'd been seeking medical help for an alcohol problem that he'd long publicly denied.

Mr Kennedy said he'd been "inundated" with messages of support from ordinary party members following that statement, but conceded that he no longer had enough support at Westminster.

"It is clear now that such support is not reflected strongly enough across the parliamentary party in the House of Commons itself." he told reporters.

"The interests of our party have to come first.

"That is where my personal, my political and my constitutional duty lies.

"So accordingly I'm announcing today that when nominations do open for the leadership of the party I will not be putting my name forward. I am standing down as leader with immediate effect."

Mr Kennedy's position become untenable yesterday, when 25 Liberal Democrat MPs announced they intended to quit the frontbench team if Mr Kennedy remained leader.

Thanking his wife and staff for sustaining him in politics since joining the Commons as the youngest MP back in 1983, he claimed he was not emotional about his departure.

"I have been in politics for far too long to be overly sentimental about this sort of moment," he said.

Despite the collapse of his support in parliament with MPs and senior colleagues briefing against him, Mr Kennedy promised whoever was elected would receive his full support.

"That new leader can be assured of my loyal support as a backbench Liberal Democrat MP," he said.

His deputy, Sir Menzies Campbell, who had promised he would not stand against Mr Kennedy, immediately declared himself a candidate in the leadership election.

Speaking to reporters at his home in Edinburgh, Sir Menzies commended Mr Kennedy for his "dignity and courage" and for his achievements as leader of the Liberal Democrats.

"Under his leadership the party has advanced to its best position for 80 years and the party will forever be in his debt," he said.

"Today we must wish him and his family well.

"As deputy leader I will be assuming his responsibilities with immediate effect, and over the next 48 hours will obviously be consulting colleagues. But I can confirm today that I intend to be candidate in the leadership contest which is to follow."

The party's president, Simon Hughes, and its home affairs spokesman, Mark Oaten, are also thought to be preparing leadership challenges, with younger "modernisers" like David Laws and Nick Clegg also seen as potential candidates.

Mr Oaten praised Mr Kennedy's dignity but refused to be drawn immediately on whether he would stand for the leadership.

"After all the pressure that Charles has had, that was a dignified, statesmanlike performance."

Asked if he would run for the leadership, he said: "What I want to do is find the best way forward to unite this party.

"I need to speak to Ming and Simon and other contenders, and see if we can't decide how best to move this party forward. It's not a time for individual ambitions, we need to reunite this party."

Tonight the party's environment spokesman, Norman Baker - one of the MPs who'd called on Mr Kennedy to quit - called on Sir Menzies to be crowned leader.

"For my part I believe the best solution now is for Menzies Campbell to take over and if that was shared by the parliamentary at large, I would not be unhappy about that," he said.

"It is obviously a sad day for Charles in particular, and for the party at large. It is inevitable that Charles had to go given the circumstances. But it doesn't take away from him his immense contribution over the last six years."

The Liberal Democrat federal executive committee is due to meet on Monday to draw up a timetable for the leadership election, and it is anticipated that a two-week nomination period will be adopted.

Under the party's leadership rules, candidates must have the support of at least seven MPs as well as 200 party activists. The contest itself will be conducted under a single transferable vote system.

Addressing the internal debate in the party between the younger economic liberals and social liberals like Simon Hughes, Mr Kennedy gave some parting words of advise.

"As I have acknowledged before, there is a genuine debate going on within this party, somewhat crudely caricatured at times as being in rather redundant terms as between left and right; in rather simplistic terms as between social liberals and economic liberals; in rather misleading terms as between traditionalists and modernisers.

"I have never accepted that these are irreconcilable instincts. Indeed, quite the opposite, and I believe that unity remains fundamental to our further advance and success."

In a implicit reference to David Cameron, whose election precipitated the leadership crisis in the Liberal Democrats, Mr Kennedy said the party should not be pushed off course by the new Tory leader.

"It must not be allowed to become dictated by others who do not share our long-term hopes and goals. We must stand and argue - politically independent and intellectually self-confident.

"And it must be based on time-honoured, sound philosophic liberal principles - principles which have stood the test of generations and remain not just as relevant to, but even more essential, in British politics today."

As Mr Kennedy arrived back at his south London home with his wife Sarah after giving his statement, she smiled and told reporters: "I'm very proud of him and what he's achieved with the party."